Part of me simply thinks it fulfills a human need to love and be loved unconditionally, which would explain why one of the signs of sociopaths is cruelty to animals at an earlier age, but that doesn’t explain the cultural divergences.

Modern psychiatrists prefer to call sociopaths "people suffering from anti-social personality disorder," though as a Szaszian aside, I'd say that if anyone "suffers" from it, it's the people who don't have it!